The present disclosure relates to a storage element, which changes the direction of magnetization of a storage layer by injecting spin-polarized electrons, and a memory having the storage element and is suitable for being applied to a non-volatile memory.
In an information apparatus, such as a computer, a DRAM with a high operation speed and a high density is widely used as a random access memory.
However, since the DRAM is a volatile memory in which information disappears when the power is turned off, a non-volatile memory in which information does not disappear is desired.
In addition, a magnetic random access memory (MRAM) which records information by magnetization of a magnetic substance has been drawing attention as a candidate for a non-volatile memory and is under development.
In the MRAM, recording of information is performed by applying a current to two kinds of address lines (word line and bit line), which are almost perpendicular to each other, and reversing the magnetization of a magnetic layer of a magnetic storage element, which is located at the intersection of the address lines, by the current magnetic field generated from each address line.
A schematic view (perspective view) of a normal MRAM is shown in FIG. 10.
A drain region 108, a source region 107, and a gate electrode 101 which form a selection transistor for selecting each memory cell are formed in portions separated by an element separation layer 102 of a semiconductor base 110, such as a silicon substrate.
In addition, a word line 105 which extends back and forth in the drawing is provided above the gate electrode 101.
The drain region 108 is formed so as to be common to left and right selection transistors in the drawing, and a wiring line 109 is connected to the drain region 108.
In addition, a magnetic storage element 103 having a storage layer in which the direction of magnetization is reversed is disposed between the word line 105 and a bit line 106 which is disposed above the word line 105 and extends left and right in the drawing. The magnetic storage element 103 is formed by a magnetic tunnel junction element (MTJ element), for example.
In addition, the magnetic storage element 103 is electrically connected to the source region 107 through a horizontal bypass line 111 and a vertical contact layer 104.
Information can be recorded by applying the current magnetic field to the magnetic storage element 103 by applying a current to each of the word line 105 and the bit line 106 so that the direction of magnetization of the storage layer of the magnetic storage element 103 can be reversed.
Moreover, in a magnetic memory, such as the MRAM, it is necessary for a magnetic layer (storage layer) which records the information to have a fixed coercive force in order to stably hold the recorded information.
On the other hand, in order to rewrite the recorded information, it is necessary to apply a certain amount of current to the address line.
However, since the address line becomes thinner as elements which form the MRAM become miniaturized, it becomes impossible to apply a sufficient current.
Therefore, a memory with a configuration in which magnetization reversal caused by spin transfer is used has been drawing attention as a configuration in which the magnetization reversal is possible with a smaller amount of current (for example, refer to Patent Documents 1 and 2 and Non-patent documents 1 and 2).
The magnetization reversal caused by spin transfer means that spin-polarized electrons having passed through the inside of a magnetic substance are injected into another magnetic substance in order to cause magnetization reversal in another magnetic substance.
For example, by applying a current to a giant magnetoresistive effect element (GMR element) or a magnetic tunnel junction element (MTJ element) in a direction perpendicular to the layer surface, the direction of magnetization of at least parts of magnetic layers of the elements can be reversed.
In addition, the magnetization reversal caused by spin transfer is advantageous in that the magnetization reversal can be realized without increasing a current even if the element becomes miniaturized.
A schematic view of a memory with a configuration in which the above-described magnetization reversal caused by spin transfer is used is shown in FIGS. 8 and 9. FIG. 8 is a perspective view, and FIG. 9 is a sectional view.
A drain region 58, a source region 57, and a gate electrode 51 which form a selection transistor for selecting each memory cell are formed in portions separated by an element separation layer 52 of a semiconductor base 60, such as a silicon substrate. Among these, the gate electrode 51 also serves as a word line which extends back and forth in FIG. 8.
The drain region 58 is formed so as to be common to left and right selection transistors in FIG. 8, and a wiring line 59 is connected to the drain region 58.
In addition, a storage element 53 having a storage layer in which the direction of magnetization is reversed by spin transfer is disposed between the source region 57 and a bit line 56 which is disposed above the source region 57 and extends left and right in FIG. 8.
The storage element 53 is formed by a magnetic tunnel junction element (MTJ element), for example. In the drawing, 61 and 62 indicate magnetic layers. It is assumed that one of the two magnetic layers 61 and 62 is a magnetization fixed layer, in which the direction of magnetization is fixed, and the other magnetic layer is a free magnetization layer in which the direction of magnetization changes, that is, a storage layer.
In addition, the storage element 53 is connected to the bit line 56 and the source region 57 through upper and lower contact layers 54, respectively. Accordingly, by applying a current to the storage element 53, the direction of magnetization of the storage layer can be reversed by spin transfer.
Such a memory with a configuration in which the magnetization reversal caused by spin transfer is used also has a feature that the device structure can be simplified compared with the normal MRAM shown in FIG. 10.
In addition, since the magnetization reversal caused by spin transfer is used, it is advantageous in that the write current does not increase even if the element is miniaturized compared with a normal MRAM which performs the magnetization reversal by the external magnetic field.
Meanwhile, in the case of the MRAM, wiring lines (word line and bit line) are provided separately from the storage element and writing (recording) of information is performed by the current magnetic field generated by applying a current to the writing lines. Accordingly, a sufficient amount of current required for writing can flow through the writing lines.
On the other hand, in the memory with a configuration in which the magnetization reversal caused by spin transfer is used, it is necessary to reverse the direction of magnetization of the storage layer by performing the spin transfer using a current flowing through the storage element.
In addition, writing (recording) of information is performed by directly applying a current to the storage element as described above. Accordingly, in order to select a memory cell which performs writing, a storage element is connected with a selection transistor to thereby form a memory cell. In this case, a current flowing through the storage element is limited to the size of a current (saturation current of a selection transistor) which can be applied to the selection transistor.
For this reason, it is necessary to perform the writing with a current equal to or less than the saturation current of the selection transistor. Accordingly, it is necessary to improve the efficiency of spin transfer so that the current flowing through the storage element can be reduced.
In addition, in order to enlarge a read signal, it is necessary to ensure a large magnetoresistance change rate. In order to do so, it is effective to adopt a configuration of a storage element in which an intermediate layer in contact with both sides of a storage layer is made as a tunnel insulating layer (tunnel barrier layer).
In the case of using the tunnel insulating layer as an intermediate layer, the amount of current flowing through the storage element is limited in order to prevent dielectric breakdown of the tunnel insulating layer. Also from this point of view, it is necessary to suppress a current at the time of spin transfer.
[Non-Patent Document 1] Phys. Rev. B 54. 9353 (1996)
[Non-Patent Document 2] J. Magn. Mat. 159. L1 (1996)
[Patent Document 1] Japanese Unexamined Patent Application Publication 2003-17782
[Patent Document 2] U.S. Pat. No. 6,256,223
However, it cannot become a memory if it does not store and hold the information written by a current. Therefore, it is necessary to ensure stability (thermal stability) against heat fluctuations of the storage layer.
In the case of the storage element that uses the magnetization reversal caused by spin transfer, the volume of a storage layer becomes small compared with that of a conventional MRAM; put simply, the thermal stability tends to fall accordingly.
If the thermal stability of the storage layer is not ensured, the reversed direction of magnetization is reversed again by heat and this becomes a writing error.
For this reason, in the storage element that uses the magnetization reversal caused by spin transfer, thermal stability is a very important characteristic.
In general, in the case of an element which seldom spends energy in writing, the information disappears easily because the energy barrier is low.
On the other hand, in the case of an element which needs large energy in writing, it is possible to form a high energy barrier. Accordingly, it can be said that storage of information is also stable.
In a storage element that uses the magnetization reversal caused by spin transfer, the thermal stability becomes high and a large amount of current is required for writing as the amount of saturation magnetization of a storage layer and the volume of the storage layer increase when the comparison is performed in the configuration where the spin transfer efficiency is equal.
Generally, a thermal stability index may be expressed by a thermal stability parameter (Δ).
Δ is given as Δ=KV/kT (K: anisotropy energy, V: volume of a storage layer, k: Boltzmann's constant, T: temperature).
Therefore, in order that a storage element with a configuration in which the direction of magnetization of a storage layer is reversed by spin transfer can exist as a memory, it is necessary to improve the spin transfer efficiency so that a current required for the magnetization reversal can be reduced so as to be equal to or smaller than a saturation current of a transistor and to ensure the thermal stability for reliably holding the written information.
Therefore, it is desired to provide a storage element capable of improving the thermal stability without increasing a write current and a memory with a storage element.